Optical spectroscopy is a very powerful analytical technique. It allows the identification of substances and materials, their quantification, and the measurements of physical parameters.
Most optical spectroscopy equipment has a dispersive element such as a prism, a diffraction grating or a variable filter as the core light-analyzing element, coupled to suitable optics (e.g., slits, mirrors, and/or lenses). These dispersive elements take broadband light as an input and separate its different wavelengths spatially. Dispersive spectroscopy equipment, such as those based on diffraction gratings, can achieve very high resolutions. However, spatial wavelength separation traditionally requires the rotation of some elements in the optical path in order to analyze the subject light. More recently, optical sensors such as complementary metal-oxide semiconductor (CMOS) pixel arrays have been used to capture a wide segment of the spatial output, and the desired wavelength is selected electronically. The main disadvantages of these techniques are the relatively large size of the rigs, the need for precise optical alignment, the presence of multiple optical surfaces susceptible to fouling, and other factors such as price. Miniaturization of some elements has allowed the reduction of size of some spectrometric solutions down to the centimeter scale, but usually this scale reduction implies a trade-off in resolution and sensitivity.
Other optical spectrometers use color filters to break up the subject light into a number of discrete “bins” of light, each bin comprising a certain region of the optical spectrum. The resolution of these systems depends on the bandwidth of the bins, and the number of different spectral bins. In general, the resolution of filter-based spectrometers is lower than those using dispersive elements. Therefore, filter-based solutions have typically been implemented where the resolution requirements are not too stringent.
Other techniques based on recent developments have appeared in the literature, such as surface plasmon resonance and graphene absorbers. However, these techniques are in the development phase and no commercial applications have been found yet.
There is a need for a compact device that can provide spectral information from a subject light without the shortcomings of present solutions. In particular, it is desirable to provide a device with electronic tuneability, so that there is no need for dispersive element(s) or mechanical adjustments. It is also desirable to provide the device in a small size, permitting the integration in portable equipment and products such as handheld medical devices, tablets and mobile phones.
Ballistic electrons have been utilized in prior art devices. For example, U.S. Pat. No. 4,286,275 (Heiblum, published Aug. 25, 1981) and U.S. Pat. No. 4,833,517 (Heiblum et al., published May 23, 1989) describe embodiments of ballistic electron transistors. However, such devices have specific structures and parameters for use as transistors, and have not been adapted to perform as an optical spectrometer.
In some implementations, in contrast to the above mentioned transistors employing ballistic carrier transport, WO 2015/069367A2 (Perera et al., published May 14, 2015) presents a tuneable hot-carrier photodetector that exploits hot-hole dynamics, but does not use ballistic transport, instead relying on other structural features and a double absorption step to analyze the subject light.